1. Field of the Invention
The present invention relates to an optical glass for a polarizing optical system which is suitably usable for a polarizing optical system such as polarizing beam splitter and spatial light modulator for effecting polarizing modulation, and has an extremely small photoelastic constant, a process for producing such an optical glass for polarizing optical system, and a polarizing beam splitter utilizing the optical glass for polarizing optical system.
2. Related Background Art
In recent years, the utilization of a xe2x80x9cpolarizing characteristicxe2x80x9d, as one of the factors constituting optical information, has rapidly been developed in various fields such as the field of liquid crystal. Along with such development in the utilization of the polarizing characteristic, in an optical system utilizing polarized light, i.e., a polarizing optical system, the importance of high-precision control of the polarizing characteristic constituting optical information has been increased year by year. Based on the increase in the above-mentioned importance, it has earnestly been desired to further improve the precision or accuracy in the control of the polarizing characteristic.
Among various optical elements constituting a polarizing optical system (such as substrate and prism), it is usual to use a material having an optical isotropy especially for some optical elements which are required to retain the polarizing characteristic. The reason for this is that when an optical element comprising a material having an optical anisotropy is used, the phase difference (optical path difference) between the ordinary ray and the extraordinary ray perpendicular to the ordinary ray will be changed during their passage through such a material, with respect to light which has been transmitted by the optical element, and therefore the polarizing characteristic cannot be retained in such a case.
In general, a glass which has sufficiently been subjected to annealing has an optical isotropy and also has various characteristics better than those of other materials in view of its durability, strength, transmittance, refractive index, cost, etc., and therefore such a glass is widely used for optical elements which should retain the polarizing characteristic. Particularly, borosilicate glass (e.g., a borosilicate glass mfd. by Schott Co., Germany, trade name: xe2x80x9cBK7xe2x80x9d) is inexpensive and excellent in durability, and also has little dispersion. Therefore, the borosilicate glass is widely used in many polarizing optical systems.
However, even when the above-mentioned conventional optical glass for polarizing optical system is used for the optical elements, a certain optical anisotropy based on a photoelastic effect can be induced in the optical element, under the application of a mechanical external stress or a thermal stress to the optical element. Accordingly, when the conventional optical glass is used for the optical element for a polarizing optical system, the polarizing characteristic of optical information can be changed on the basis of the xe2x80x9cinduced optical anisotropyxe2x80x9d as described above. Therefore, in such a case, it is difficult for the polarizing optical system to exhibit a desired performance.
It is considered that the mechanical external stress and the thermal stress as described above are developed mainly in the following situation.
Thus, it is considered that the xe2x80x9cmechanical external stressxe2x80x9d is mainly developed in a step of processing a glass (such as cutting, the bonding or joining of the glass with another material, and film formation on the surface of a glass), or often a step of assembling a glass into an optical system (such as holding of the glass by a jig or holding device, and the adhesion of the glass to another member). In addition, it is considered that the xe2x80x9cthermal stressxe2x80x9d is developed by the production of heat in the interior of a glass (such as heat production based on the absorption of light energy), or the production of heat outside a glass (e.g., that based on heat production in a peripheral device). Further, when a glass is caused to contact or is joined with another material having a thermal expansion coefficient different from that of the glass, it is considered that a stress is developed along with the above-mentioned production of heat.
As described above, when a polarizing optical system is constituted by using an optical element, it has been difficult to completely obviate the action of the mechanical external stress or the thermal stress. Accordingly, when the conventional optical glass for polarizing optical system is used for such an optical system, it is extremely difficult to avoid the induction of the optical anisotropy based on the above-mentioned mechanical external stress or thermal stress.
An object of the present invention is to provide an optical glass for polarizing optical system, which does not substantially impair the polarizing characteristic of optical information, even under the action of a mechanical external stress or a thermal stress.
Another object of the present invention is to provide an optical glass for polarizing optical system, which is capable of controlling its refractive index in a desirable manner.
As a result of earnest study, the present inventors have found that the polarizing characteristic of optical information in an optical glass for polarizing optical system (under the action of a mechanical external stress or a thermal stress) may desirably be evaluated by using a xe2x80x9cphotoelastic constant based on the value of birefringence or double refraction (under the application of a stress) measured by a photoelasticity modulation methodxe2x80x9d. The optical glass for polarizing optical system according to the present invention is based on the above discovery and characterized by a photoelastic constant C thereof in the range of xe2x88x920.2 to +0.5 [10xe2x88x928 cm2/N] with respect to a wavelength of 633 nm.
An optical glass for polarizing optical system according to the present invention has a photoelastic constant C in the range of xe2x88x920.2 to +0.5 [10xe2x88x928 cm2/N] with respect to a wavelength of 633 nm, the optical glass having the following composition (1):
composition (1): when represented in terms of wt. % of oxides:
SiO2: 17.0-27.0% (35.5-57.0 mol %)
Li2O+Na2O+K2O: 0.5-5.0% (0.7-20.0 mol %)
PbO: 72.0-75.0% (39.1-45.0 mol %)
As2O3+Sb2O3: 0.1-3.0% (0.1-2.0 mol %).
Another optical glass for polarizing optical system according to the present invention has a photoelastic constant C in the range of xe2x88x920.2 to +0.5 [10xe2x88x928 cm2/N] with respect to a wavelength of 633 nm, the optical glass having the following composition (2):
composition (2): when represented in terms of mol %:
SiO2: 40.0-54.0 mol %
R2O (R: alkali metal): 0.5-9.0 mol %
PbO: 43.0-45.5 mol %
As2O3+Sb2O3: 0.1-1.5 mol %; and
the composition (2) further containing fluorine in the following range when represented in terms of mol %:
fluorine/oxygen (F/O) ratio: 0.1-18.0.
A further optical glass for polarizing optical system according to the present invention has a photoelastic constant C in the range of xe2x88x920.2 to +0.5 [10xe2x88x928 cm2/N] with respect to a wavelength of 633 nm, the optical glass having the following composition (3):
composition (3) when represented in terms of mol %:
SiO2: 40.0-54.0 mol %
R2O (R: alkali metal): 0.5-9.0 mol %
RF: 0-16.0 mol %
R2SiF6: 0-3.3 mol %
PbO+PbF2: 43.0-45.5 mol %
PbF2: 0-10.0 mol %
As2O3+Sb2O3: 0.1-1.5 mol %; and
the composition (3) further containing fluorine in the following range in terms of mol %:
fluorine/oxygen (F/O) ratio: 0.1-18.0.
A process for producing an optical glass for polarizing optical system according to the present invention comprises:
changing the ratio of PbO in a lead-containing optical glass to control the photoelastic constant C thereof to provide an optical glass for polarizing optical system having a photoelastic constant C in the range of xe2x88x920.2 to +0.5 [10xe2x88x928 cm2/N] with respect to a wavelength of 633 nm, the optical glass having the following composition (1):
composition (1): when represented in terms of wt. % of oxides:
SiO2: 17.0-27.0% (35.5-57.0 mol %)
Li2O+Na2O+K2O: 0.5-5.0% (0.7-20.0 mol %)
PbO: 72.0-75.0% (39.1-45.0 mol %)
As2O3+Sb2O3: 0.1-3.0% (0.1-2.0 mol %).
Another process for producing an optical glass for polarizing optical system according to the present invention comprises:
changing the fluorine/oxygen (F/O) ratio of a fluorine-containing optical glass so as to regulate the refractive index thereof while retaining the photoelastic constant C of the optical glass in the range of substantially zero to provide an optical glass for polarizing optical system having a photoelastic constant C in the range of xe2x88x920.2 to +0.5 [10xe2x88x928 cm2/N] with respect to a wavelength of 633 nm,
the optical glass having the following composition (2):
composition (2): when represented in terms of mol %:
SiO2: 40.0-54.0 mol %
R2O (R: alkali metal): 0.5-9.0 mol %
PbO: 43.0-45.5 mol %
As2O3+Sb2O3: 0.1-1.5 mol %; and
the composition (2) further containing fluorine in the following range when represented in terms of mol %:
fluorine/oxygen (FIO) ratio: 0.1-18.0.
A further process for producing an optical glass for polarizing optical system according to the present invention comprises:
changing the fluorine/oxygen (F/O) ratio of a fluorine-containing optical glass so as to regulate the refractive index thereof while retaining the photoelastic constant C of the optical glass in the range of substantially zero to provide an optical glass for polarizing optical system having a photoelastic constant C in the range of xe2x88x920.2 to +0.5 [10xe2x88x928 cm2/N] with respect to a wavelength of 633 nm,
the optical glass having the following composition (3):
composition (3) when represented in terms of mol %:
SiO2: 40.0-54.0 mol %
R2O (R: alkali metal): 0.5-9.0 mol %
RF: 0-16.0 mol %
R2SiF6: 0-3.3 mol %
PbO+PbF2: 43.0-45.5 mol %
PbF2: 0-10.0 mol %
As2O3+Sb2O3: 0.1-1.5 mol %; and
the composition (3) further containing fluorine in the following range in terms of mol %:
fluorine/oxygen (FIO) ratio: 0.1-18.0.
In general, when a force is applied to a transparent substance having homogeneity and isotropy such as glass so as to develop a stress therein, an optical anisotropy is induced in the transparent substance, and the transparent substance is caused to have a birefringence property in a similar manner as in a certain kind of crystalline substance. Such a phenomenon is called an xe2x80x9cphotoelastic effectxe2x80x9d. The refractive index of a transparent substance in which a stress has been developed, may be represented by a so-called xe2x80x9c(refractive) index ellipsoidxe2x80x9d, and the principal refractive index axis of the refractive index ellipsoid coincides with the principal stress axis.
In general, when the principal refractive indices are denoted by n1, n2, and n3, and the principal stresses are denoted by "sgr"1, "sgr"2, and "sgr"3 (those having the common subscript are those having the same direction), these principal refractive indices and principal stresses satisfy the following relationship.
n1=n0+C1"sgr"1+C2("sgr"2+"sgr"3) 
n2=n0+C1"sgr"2+C2("sgr"1+"sgr"1) 
n3=n0+C1"sgr"3+C2("sgr"1+"sgr"2) 
In a case where light is incident on the transparent substance having such a refractive index, when a coordinate is defined so that the direction of the incident light is the same as that of the above "sgr"3, the incident light is separated into two linearly polarized light components respectively having "sgr"1 and "sgr"2 directions (namely, linearly polarized light components respectively having planes of vibration which are perpendicular to each other). On the other hand, when light emerges from the transparent substance, in a case where the refractive index in the respective directions of the principal stresses (n1, n2) are different from each other, an optical path difference (phase difference) xcex94xcfx86 represented by the following equation is provided between these two linearly polarized light components.                                                         Δφ              =                                                (                                      2                    ⁢                                          π                      /                      λ                                                        )                                ⁢                                                      (                                                                  n                        2                                            -                                              n                        1                                                              )                                    ·                  l                                                                                                        =                                                (                                      2                    ⁢                                          π                      /                      λ                                                        )                                ⁢                                  (                                                            C                      1                                        -                                          C                      2                                                        )                                ⁢                                                      (                                                                  σ                        2                                            -                                              σ                        1                                                              )                                    ·                  l                                                                                                                                          =                                                                                    (                                                  2                          ⁢                                                      π                            /                            λ                                                                          )                                            ·                      C                      ·                                              σ                        2                                                              -                                          σ                      1                                                                      )                            ·              l                                                          ⟨                  Equation          ⁢                      xe2x80x83                    ⁢          2                ⟩            
In the above Equation 2, xcex denotes the wavelength of light, and l (xe2x80x9celxe2x80x9d) denotes the light transmission thickness of the transparent substance. The constant C=C1-C2 in the above Equation is called xe2x80x9cphotoelastic constantxe2x80x9d.
According to the present inventor""s knowledge, the value of the photoelastic constants C of conventional optical glasses which have been used for polarizing optical systems are large. For example, the value of the above constant C=2.78 [10xe2x88x928 cm2/N] (wavelength xcex=633 nm) was obtained in the case of the commercially available borosilicate glass xe2x80x9cBK7xe2x80x9d (Schott Co.) as described hereinabove. In the case of the borosilicate glass having such a large photoelastic constant C, the optical anisotropy induced by the thermal stress or mechanical external stress, and the optical path difference xcex94xcfx86 based on the anisotropy, naturally become certain values which are not negligible.
On the contrary, in the case of the above-mentioned optical glass for polarizing optical system according to the present invention, the photoelastic constant C is in the range of substantially zero, with respect to a wavelength of 633 nm. The term xe2x80x9ca photoelastic constant C in the range of substantially zeroxe2x80x9d used herein refers to a condition such that the influence of the optical path difference due to optical anisotropy, which is provided when the glass is used for a polarizing optical system, is within a negligible extent with respect to the entirety of the above optical system. The photoelastic constant C is in the range of xe2x88x920.2 to +0.5 (preferably xe2x88x920.1 to +0.3) [10xe2x88x928 cm2/N] with respect to incident light having a wavelength of 633 nm.
FIG. 1 is a graph showing a relationship between the fluorine/oxygen (F/O) ratio in a composition of the optical glass for polarizing optical system according to the present invention wherein the photoelastic constant C becomes substantially zero for a wavelength of incident light (633 nm), and the refractive index of the glass. Further, FIG. 2 is a graph showing variation in the photoelastic constant C along with a change in the above F/O ratio in the above-mentioned glass composition.
As shown in FIGS. 1 to 2, in the refractive index of the optical glass according to the present invention, a certain linearity may be established with respect to the F/O ratio, and it is observed that the photoelastic constant C of the glass becomes substantially zero irrespective of the F/O ratio. According to the present inventors"" knowledge, the photoelastic constant C is dependent on the lead ion content in the optical glass but is not dependent on the amount of fluorine ions introduced into the glass, and therefore it is assumed that a phenomenon such that the photoelastic constant C becomes substantially zero is established in the glass composition according to the present invention.
FIG. 3 is a graph showing transmission spectra of one composition series of the optical glass according to the present invention at a depth (thickness) of 10 mm. As shown in FIG. 3, it is recognized that the transmittance of blue light is increased by introducing fluorine into a glass composition. According to the present inventors"" investigation, it is recognized that the tendency of an increase in the blue light transmittance becomes marked as the F/O ratio is increased, and along with such an increase, the absorption edge (limit of absorption on the shorter wavelength side) is also shifted to the shorter wavelength side.
Further, As2O3 and/or Sb2O3 is essentially contained in the optical glass according to the present invention in an amount of 0.1 to 3.0 wt. % (0.1-2.0mol %). Since the optical glass according to the present invention contains As2O3 and/or Sb2O3 which is capable of functioning as a defoaming agent in amount of 0.1 to 3.0 wt. % (0.1-2.0 mol %), quite a high internal transmittance with respect to light having a wavelength of 400 nm or more can be achieved.